Acid treatment of pulp at high temperature in connection with bleaching

ABSTRACT

In the treatment of cellulose pulp in connection with bleaching, the pulp is first cooked and, if necessary, oxygen delignified in order to reduce the Kappa number below 24, preferably below 14, and after that the pulp is treated in an acid stage at a pH of 2-5 and at a temperature range of 75-130° C. in order to reduce the Kappa number by 2-9 units. To prevent essential weakening of the strength properties of the pulp in the acid stage treatment tower, the retention time, t min, is 30 to 300 minutes and the treatment temperature, T° C. is T min &lt;T&lt;T max , in which  
         T   min     =       10517     24   +     ln        (     2      t     )           -   273                   
 
     and  
     
       T 
       max 
       =T 
       min 
       +23 
       ° C.

[0001] The present invention relates to a method and apparatus fortreating pulp at a pH of 2-5 and at a temperature of 75-130° C. inconnection with bleaching,

[0002] Chemical pulp has been produced conventionally by first digestingpulp usually with sulphate or sulphite cooking liquor either in acontinuous or a batch digestion process, by delignifying and screeningthe pulp before a bleaching process. The bleaching was still in the1980's commonly done by using chlorine dioxide.

[0003] In the 1990's efforts have been made to replace chlorine dioxidewith oxygen, ozone and/or peroxide because environmental factors forcepulp mills to employ either totally chlorine-free or at least elementalchlorine-free processes. Sequences using chlorine dioxide are, however,still popular in many countries and possible also in view of theenvironment. There are many reasons for this. The price of chlorinedioxide is very competitive compared to other chemicals; for example itsprice is today only half of the price of the competing peroxide. Alsothe strength properties and brightness of the pulp produced by chlorinedioxide bleaching are good; in fact at least of the same order as whenusing peroxide at the same consumption of chemical (kg/adt).

[0004] When the bleaching of cellulose pulp is based on the use of suchbleaching chemicals as oxygen, peroxide, or ozone, the removal of heavymetals forms an essential process stage. Detrimental metals includemanganese, copper and iron, which catalyze reactions harmful to thequality of pulp. They degrade bleaching chemicals, which decreases theefficiency of bleaching and increases the consumption of chemicals. Ithas been suggested that removal of metals be effected by pretreating thepulp prior to the critical bleaching stage with an acid, e.g. sulphuricacid whereby metals are detached from fibers to the liquid phase in thepulp. Before the bleaching stage itself the pulp is still washed bydisplacing the liquid in the pulp by cleaner wash liquid.

[0005] In acid treatments which are used to remove metals thetemperature is usually not of significant importance. The most essentialfeature is that the pH of the pulp is so low (usually 1.5-2) that metalsare detached from fibers. In laboratories the treatment is usuallycarried out at room temperature. In mills the metal removal is typicallyeffected at a temperature range of 60-85° C., which is the temperaturenaturally prevailing in the acid treatment stage as a consequence ofwater circulations. If the mill would for some reason want to effect theacid treatment at a higher temperature the acid stage would have to beheated separately by steam or by a corresponding method. Naturally, thishas been avoided because of the belief that the strength properties ofthe pulp would impair. Based on the state of the art knowledge there hasbeen no reason to use hot acid stages.

[0006] It has been discovered recently that sulphate pulps containsignificant amount of 4-deoxy-β-L-threo-hex-4-enopyranosyl uronic acidgroups (hexenuronic acid groups) sound to xylan. Further, it has beenfound out (patent application wo 96/12063) that by removing hexenuronicacid groups from the pulp its Kappa number may be decreased remarkably.These acids are removed by adjusting the pH of the pulp to the range of2-5, preferably 2.5-4 and the temperature of the pulp preferably to90-110° C. and allowing these conditions to act on the pulp for acertain time and subsequently the pulp has been washed according to thepublications mentioned above. By this kind of removal of hexenuronicacids, significant saving are obtainable in the consumption of bleachingchemicals which react electrophilically, such as ozone, peracids andchlorine dioxide, because they react with these acid groups.

[0007] When the acid treatment mentioned is used in connection withperoxide bleaching the brightness reversion of the pulp has been foundto decrease.

[0008] As was stated above until now acid treatments at hightemperatures have consciously been avoided as they have been believed tohave a negative effect on the strength properties of the pulp.

[0009] An object of the present invention is to provide a method ofcarrying out acid treatment at a high temperature so that the strengthproperties of the pulp do not degrade during the treatment.

[0010] The characteristic features of the method and the apparatus ofthe inveniton are disclosed in the appended patent claims.

[0011] According to the method of the present invention the pH of thepulp, which has been delignified during digestion and possibly oxygendelignified after that and having a Kappa number of less than 24,preferably less than 14, is adjusted to a slightly acid range, e.g. 2-5,preferably 2.5-4. The temperature of the pulp is raised to 75-130° C.,preferably 90-110° C. Then the Kappa number of the pulp reduced by 2-9units, preferably 3-6 units. In order to obtain this, a retention timeof 30-300 minutes, preferably 45-150 minutes, is required. The acidtreatment may be effected at atmospheric pressure or as a pressurizedprocess depending on the conditions. A preferred pressure is 0.1-1 MPa.At least 30%, preferably 50% of the hexenuronic acids contained in thepulp are degraded in this way.

[0012] One of the central ideas of the present invention is that theacid treatment is effected by minimizing the treatment temperatureand/or treatment time of the pulp in order to optimize the strengthproperties of the pulp. It has been discovered that an adequate decreasein Kappa number and removal of hexenuronic acids is obtained when thetreatment temperature is at least T_(min)° C.$T_{\min} = {\frac{10517}{24 + {\ln \left( {2t} \right)}} - 273}$

t=retention time, min

[0013] The equation has been presented graphically in FIG. 1. Thedegradation of hexenuronic acid groups follows the reaction kinetics ofthe first order. It is known that the dependency between the affinityconstant k and the temperature T (K) is k=A e^(-E/RT) (equation ofArrhenius) in which A is constant depending on the reaction, E isactivation energy and R is gas constant. On the other hand it is knownthat the reaction time for the reaction of the first order is t=(1/k)ln(c₀/c) in which c is the hexenuronic acid content and c₀ the initialcontent. By using the Arrhenius equation and the equationt=(1/k)ln(c₀/c) and test results, the above T_(min) expression ensues.Further, it has now been discovered that in order to prevent thestrength properties of the pulp from being deteriorated the highestemployable treatment temperature T_(max)° C. is$T_{\max} = {\frac{10517}{24 + {\ln \left( {2t} \right)}} - 250}$

[0014] On the other hand, it may be stated that

T _(max) =T _(min)+23° C.

[0015] According to a particular aspect of the invention high qualitypulp is produced by adjusting the acid treatment temperature to a rangeof T_(min)-T_(max) while the retention time is 30-300 minutes. In FIG.1, the lower curve illustrates T_(min) and the upper one T_(max).Favourable treatment conditions lie in the area between these. Typicaloperation conditions are the depicted by points A (90° C., 180 min), B(95° C., 120 min), and C (100° C., 70 min).

[0016] In order to preserve the pulp strength properties and to obtainhomogenous pulp by acid treatment it is essential that the flow of pulpthrough the acid tower is smooth and no portions of pulp remain in thetower for example due to channelling for a longer or for a shorter timethan the predetermined retention time. This may be prevented by usingmeans controlling the feed and the discharge of pulp in an upward flowtower and means for controlling the level of pulp in a downward flowtower. The feed members must be able to distribute the pulp evenly overthe whole cross-sectional area of the pulp tower. The optimization oftreatment conditions may be facilitated by using two or more successiveacid towers which makes it possible to act on the treatment conditions,for example by regulating the temperature or the pH, or by addingchemicals which are advantageous for the treatment, e.g. chelatingagents to remove metals from the pulp, or merely by mixing the pulp.

[0017] By means of the invention, it is possible to manufacture easilybleachable cellulose pulp by means of a sulphate method or an equivalentalkalic method bringing hexenuronic acids into the pulp. It ischaracteristic of the pulp manufactured according to the invention thatit contains a small amount of hexenuronic acids at the most and thehexenuronic acids are removed in a controlled way so that the strengthproperties of the pulp are not impaired. The pulp produced can be easilybleached without chlorine (ECF) or chlorine chemicals (TCF), or evenwith oxygen gas and/or peroxide, alone. An advantage provided by thedecomposition of hexenuronic acids is that the consumption of bleachingchemicals is substantially reduced.

[0018] Another central idea of the invention is to combine the treatmentdescribed above to another treatment, preferably to a peroxide stage oran acid bleaching stage, for example a chlorine dioxide stage, an ozonestage, a chlorine stage, or an acid peroxide stage (which is usuallyeffected with a peracid). Performed tests have surprisingly shown thatthe breakdown products of hexenuronic acids need not be washed off fromthe pulp before the following bleach treatment but the bleach treatmentmay be carried out in the subsequent treatment tower without anintermediate wash. Thus, compared with the processes described in priorpatent applications, the mill can manage with remarkably smaller washerinvestments because the washer following the acid treatment may by leftout altogether.

[0019] The method of the present invention may be applied advantageouslyin connection with the following bleaching sequences or partialbleaching sequences (in other words, the sequence may include also othertreatment stages in addition to the ones mentioned in the followingexamples):

[0020] An acid stage combined at first with chlorine dioxide bleaching

[0021] Digestion—O-AD-ZD

[0022] Digestion—O-AD_(N)-D

[0023] Digestion—O-AD-D

[0024] Digestion—O-ADQ-O_(p)-D

[0025] Digestion—O-ADQ-O_(p)-ZD

[0026] Digestion—O-ADQ-P

[0027] Digestion—O-AD-O_(p)-D

[0028] Digestion—O-AD-O_(p)-ZD

[0029] Digestion—O-AD-P, in which the AD may be performed in the orderDA and the ADQ stage in the order AQD, DAQ, DQA, QDA or QAD,

[0030] or an acid stage combined with acid peroxide bleaching

[0031] Digestion—O-AP_(a)Q-O_(p)-D,

[0032] Digestion—O-AP_(a)Q-O_(p)-ZD,

[0033] Digestion—O-AP_(a)Q-P

[0034] Digestion—O-AQ-ZP_(a)Q-P

[0035] Digestion—O-AP_(a)Q-O_(p)-ZD,

[0036] Digestion—O-P_(a)A-ZD

[0037] Digestion—O-AP_(a)-D

[0038] Digestion—O-P_(a)A-D

[0039] Digestion—O-P_(a)AQ-P

[0040] Digestion—O-AP_(a)Q-P

[0041] Digestion—O-A-P_(a)Q-P

[0042] in which AP_(a)Q may be performed in the order AQP_(a), QAP_(a),Qp_(a)A, P_(a)AQ or P_(a)QA.

[0043] If the digestion has brought the Kappa number down sufficientlythe O stage may be eliminated which gives a sequence:

[0044] Digestion—AD-ZD

[0045] Digestion—AD_(N)-D

[0046] Digestion—AD-D

[0047] Digestion—ADQ-O_(p)-D

[0048] Digestion—ADQ-O_(p)-ZD

[0049] Digestion—ADQ-P

[0050] Digestion—AD-O_(p)-D

[0051] Digestion—AD-O_(p)-ZD

[0052] Digestion—AD-P

[0053] Digestion—AP_(a)Q-O_(p)-D

[0054] Digestion—AP_(a)Q-O_(p)-ZD

[0055] Digestion—AP_(a)Q-P

[0056] Digestion—AP_(a)Q-O_(p)-ZD

[0057] Digestion—P_(a)A-ZD

[0058] Digestion—AP_(a)-D

[0059] Digestion—P_(a)A-D

[0060] Digestion—P_(a)AQ-P

[0061] Digestion—AP_(a)Q-P

[0062] Digestion—AQ-P_(a)Q-P

[0063] Digestion—AQ-ZP_(a)Q-P.

[0064] The same alternatives which were presented earlier apply also tothe sequences presented here, i.e. both the ADQ stage and the AP_(a)Qstage may be performed in a different order.

[0065] If it is desirable to bleach the pulp before a D stage with achlorine-free bleaching chemical the following sequences which mostoften use ozone are advantageous:

[0066] Digestion—O-AQ-ZDQ-P

[0067] Digestion—AQ-ZDQ-P

[0068] If it is desirable to bleach pulp totally without chlorine thefollowing sequences are favourable:

[0069] Digestion—O-AQ-ZQ-P

[0070] Digestion—O-AQ-P

[0071] Digestion—O-AQ-O-ZQ-P

[0072] Digestion—AQ-ZQ-P

[0073] Digestion—AQ-P

[0074] Digestion—AQ-O-ZQ-P

[0075] Digestion—Q-O-AP_(a)Q-P.

[0076] Further, the ZD, ZQ and ZDQ stages may be performed in adifferent order.

[0077] An acid stage combined with peroxide bleaching:

[0078] Digestion—O-Q-AP

[0079] Digestion—O-Q-PA

[0080] or an acid stage combines with acid peroxide bleaching:

[0081] Digestion—O-Q-AP_(a)

[0082] Digestion—O-Q-P_(a)A

[0083] If the digestion has brought the Kappa number of the pulp downsufficiently the 0 stage may be eliminated which gives sequences:

[0084] Digestion—Q-AP

[0085] Digestion—Q-PA

[0086] Digestion—Q-AP_(a)

[0087] Digestion—Q-P_(a)A

[0088] The following abbreviations have been used in all the abovesequences:

[0089] A=an acid treatment at a raised temperature according to thepresent invention

[0090] O=oxygen treatment

[0091] O_(p)=an oxygen treatment intensified with peroxide

[0092] D=a chlorine dioxide treatment in which DA means two successivetowers D and A without an intermediate wash. This may be marked also byD/A. Also abbreviations AD, AP, PA, AZ, ZA, AP_(a), P_(a)A, ZQ and AQindicate the corresponding situation.

[0093] D_(N)=chlorine dioxide treatment with neutralization

[0094] p=peroxide treatment

[0095] P_(a)=acid peroxide stage usually performed with peracid

[0096] P_(n)=several successive peroxide treatment stages

[0097] E=alkaline stage

[0098] Z=ozone treatment

[0099] Q=chelating agent treatment

[0100] “-”=wash between stages

[0101] All the above sequences may use either pressurized or atmosphericbleaching steps/stages. Further, the bleaching steps/stages may becarried out at a temperature of over or below 100° C. depending on thegoal of the step/stage in question. Also the oxygen (O) and peroxide (P)stages may be carried out as a one-step or a multi-step process as hasbeen described in prior patent applications. Concisely expressed, it maybe stated that the method and apparatus described in our patentapplications FI 892243, FI 924805, FI 925159, FI 925558, FI 930954, FI934036, FI 934056, FI 935784, FI 941229, FI 942341, FI 943001, FI944144, FI 944348, FI 945139, FI 950200, SE 9500573, FI 950749, FI950785, FI 951196, SE 9502078, FI 953343, Fl 953064, FI 953074 and FI953305 and in patents already granted based on these applications may beused to practise the present invention.

[0102] It is characteristic of the present invention that the acidtreatment described above is preferably carried out after oxygendelignification. However, an oxygen delignification stage is notnecessary it the Kappa number of the pulp has been brought downsufficiently in connection with the digestion, i.e. to a number lessthan 24, preferably less than 14. Instead of or in addition to theoxygen delignification stage, the further treatment of the pulp may alsobe some other delignification or bleaching stage by means of which theKappa numbers mentioned above are reached. A stage of this kind may befor example a chlorine stage, an ozone stage, or a second oxygentreatment stage.

[0103] The purpose of the chelating agent is to bind harmful metals suchas manganese from the pulp. Stages AQ and ZQ are preferably effected ina two-tower system. It should, however, be stated that the acidtreatment according to the invention also removes harmful metalsefficiently and thus the use of a chelating agent is usually notnecessary. Thus, the Q stage may be omitted from the combination AQ, ZQetc, or the method described in pending U.S. patent application Ser. No.566,665 “Alkaline treatment for metal ion removal during pulp bleaching”filed on Dec. 4, 1995 may be used to replace the Q stage or it may beused as the Q stage. The use of a chelating agent prior to a bleachingstage combined with an acid stage is, however, often advantageous inorder to bring about a suitable metal profile in the pulp beforebleaching.

[0104] Various acids, inorganic acids, for example mineral acids such assulphuric acid, hydrogen nitrate and hydrochloric acid, as well asorganic acids such as formylic acid and/or ethylic acid may be used toadjust the pH of the pulp suspension.

[0105] When an acid treatment has been combined in the same treatmentstage with peroxide treatment (AP) the utilization of the residualperoxide may cause a problem. This bleaching stage is preferablyeffected by providing in the process prior to the acid treatment tower apress, preferably a washing press, in which the pH of the pulp isdecreased to the range of 3-5 and the consistency of the pulp is raisedto the range of 20-40%. Thus the acid treatment is carried out withhigh-consistency pulp. For the peroxide step following the acidtreatment the pulp is diluted to the consistency range of 10-20% byusing the peroxide-containing filtrate from the filter following theperoxide tower. In this way the residual peroxide may be utilizedalthough according to the invention there is no intermediate washbetween the acid tower and the peroxide tower.

[0106] The method and the apparatus of the invention are described morein detail by way of example below with reference to the accompanyingdrawing figures of which

[0107]FIG. 1 illustrates graphically the optimal conditions for carryingout the invention as described above;

[0108]FIG. 2 illustrates a preferred embodiment of apparatus used incarrying out the method of the invention;

[0109]FIG. 3 illustrates a second preferred embodiment of apparatus usedin carrying out the method of the invention;

[0110]FIG. 4 illustrates a third preferred embodiment of apparatus usedin carrying out the method of the invention;

[0111]FIG. 5 illustrates a fourth preferred embodiment of apparatus usedin carrying out the method of the invention;

[0112]FIG. 6 illustrates a fifth preferred embodiment of apparatus usedin carrying out the method of the invention; and

[0113]FIG. 7 illustrates a sixth preferred embodiment of apparatus usedin carrying out the method of the invention.

[0114]FIG. 2 illustrates a way of effecting a preferred embodiment ofthe acid treatment. Pulp is transferred by a high-consistency pump 12from a previous treatment stage 10 to an acid tower 18. The consistencyof the pulp is 6 -25%, preferably 8-18%. The previous treatment stagemay be a wash following the digestion or, which is more common, anoxygen delignification stage or a wash stage following it.

[0115] Prior to the tower, acid is added to the pulp in order to adjustthe pH of the pulp to the range of 2-5. The acid and possibly otherchemical may be added directly to the pump 12, they may be injected to aduct 16 between the pump 12 and the tower, or to a mixer 14 provided forthis purpose.

[0116] In order to raise the temperature to the required level steam isadded to the pulp. Steam may be added in a steam mixer (not illustrated)prior to the pump 12 or it may be mixed in the duct 16 after the pump 12or in the particular mixed 14. Alternatively pulp may be heatedindirectly in the duct 16. Also the feed end of the tower may beprovided with heat transfer surfaces 22 to heat the pulp to the desiredtemperature.

[0117] According to the invention the pulp feed and flow to the tower ismade as even as possible by using a distributor 20 or a doctor. Adistributor of this kind has been disclosed for example in U.S. Pat. No.4,964,950 and Finnish patent application 924805 discusses its use forthe purpose mentioned. Correspondingly, the top of the tower has beenprovided with a discharger 24 or a discharge doctor. The top dischargermay be provided with gas separating means as disclosed in patentapplications WO 90/13344 and WO 93/01875 by A. AHLSTROM CORPORATION. Thepulp is discharged via line 28 for further treatment.

[0118] When the acid tower has been provided with means controlling thepulp feed and/or discharge the retention time of the whole pulp volumeflowing in the tower is even and no channeling occurs in the tower.Channelling would result in different retention times for different pulpportions and consequently part of the pulp would have to stay in thetower for a too long or a too short time. A long time in a hightemperature would deteriorate the quality of the pulp. All thefavourable effects on the pulp provided by the acid treatment are notreached in full if the treatment time is inadequate. Homogenous pulp canbe produced by using the arrangement according to the present invention.

[0119]FIG. 3 illustrates another apparatus for carrying out the acidtreatment. Also in this embodiment pulp is pumped from a previoustreatment stage 110 by a high-consistency pump 112. Acid is added in thesame way as in connection with FIG. 1. The pulp is also heated asdescribed above. An essential feature of this embodiment is that thepulp is pumped via line 116 to the top of an acid tower 118 wherefrom itflows via a distributor 120 to the tower and there downwards to thebottom of the tower. The distributor at the top evens out thetemperature differences in the pulp if steam has been added to the pulp.

[0120] There is a pulp surface 126 in the tower the level of which maybe controlled. In this way the pulp retention time in the tower may beregulated so that it is substantially constant for the pulp flowing inthe tower. Pulp is discharged from the tower by means of a doctor 130and a drop leg 124 and it is pumped further by a pump 128 for furthertreatment.

[0121]FIG. 4 illustrates an embodiment combining the tower typesillustrated in FIGS. 2 and 3. Thus the acid treatment is two-stepped.Pulp is fed from a stage 210 preceding the acid treatment by ahigh-consistency pump 212 in line 216 via a distributor 220 to a firstacid tower 218. Acid may be mixed into the pulp as described inconnection with FIG. 1. Also heating of the pulp may be similar to theone in FIG. 1.

[0122] The pulp is transferred by means of the pressure generated by thefirst acid tower 218, or in fact by the feed pump 212, to a distributor230 at the top of the second acid tower 232. Thus, pumping is notnecessary between the towers. In the tower 232 the pulp flows downwardsin the same way as in the tower embodiment illustrated in FIG. 3. Theretention of the pulp in the acid tower 232 is controlled by controllingthe pulp level 234. Pulp is discharged from the tower by means of adoctor 236 and a drop leg 238 and is transferred by a pump 240 to afollowing treatment stage.

[0123] An advantage provided by a two-step acid treatment is thatbetween the towers there still is a change for fine adjustment of thetreatment conditions such as pH, temperature or other factors in orderto optimize them. An alternative is to add in the duct 228 complexformer which binds harmful metals and at the same time to raise the pH.For this purpose a mixer 222 may be provided in the duct. Alternatively,the pulp discharge means 224 may be designed to be capable of mixingchemicals and/or raising pressure.

[0124] As may be understood from the above, a characteristic feature ofthe present invention is that acid treatment may be carried out as atwo-step or a multi-step process in successive towers in order tooptimize acid treatment conditions. Various adjustments may be madebetween the steps for example to keep the pH within the optimal range.

[0125]FIG. 5 illustrates a process arrangement according to a preferredembodiment of the invention. FIG. 5 illustrates a way of carrying outthe bleaching sequence ADQ although a tower arrangement of the similartype may be employed also in an AD stage in which case the A step iscarried out in a two-tower embodiment as illustrated in FIG. 4. FIG. 5,however, illustrates that the pump comes from a washer 310 which may bea brown stock washer, a washer following an oxygen delignificationprocess or a washer following some other treatment stage. It should beborne in mind that the stage according to the invention need not bepreceded by a washer but the preceding means may be for example astorage tower or some other tower. However, in the embodimentillustrated, pulp is pumped by a high-consistency pump 312 to a mixer314 in which acid in mixed into the pump in order to adjust the pH tothe range required by the acid treatment described above. The acid aswell as many other chemicals may well be mixed directly with a so-calledMC pump. The pulp is guided from the mixer to a distributor 316 whichensures that the pulp is evenly distributed over the wholecross-sectional area of the acid tower 318 which in turn ensures thatthe pulp column rises upwards smoothly minimizing the risk ofchanneling. The temperature of the pulp is controlled either by addingsteam when needed to the pulp in the mixer 314 or in the pipe linefollowing it, or by raising the temperature of the pulp indirectly inconnection with the pipe line or in the tower 318 itself. The tower 318has been dimensioned according to the retention time required by theacid treatment. The top of the tower 318 has been provided with a topdischarger 320 by means of which pulp is preferably discharged to amixer 322 in which chlorine dioxide and other required chemicals aremixed into the pulp.

[0126] The task of the mixer may be performed also by the top discharger320 which eliminates the need of a separate mixer. The pulp istransferred from the mixer to a D tower 324 either via one feedconnection as illustrated or via several feed connections by applyingthe distributor 316 illustrated in connection with the tower 318. Asdescribed in connection with the tower 318, the pulp is discharged fromthe tower 324 by means of a top discharger 326 in which chelatingagainst required by the subsequent step may be mixed into the pulp orgas may be separated from the pulp. If it is desirable to employ aseparate mixer 328 it may be installed preceding the Q tower 330 and thedischarge therefrom may be effected as already described.

[0127]FIG. 6 illustrates a process arrangement according to anotherpreferred embodiment of the invention. The figure illustrates anapparatus diagram for example for an AP_(a)Q stage in which the acidperoxide step, i.e. the P_(a) step, is carried out in two towers. Asdescribed above the pulp is fed by means of a high-consistency pump 342from a preceding apparatus 340 to a mixer 344 in which acid, and ifdesired also steam, is mixed into the pulp. The pulp is guidedpreferably via a distributing feed apparatus 346 to an A tower and fromthis tower the pulp is removed via a top discharger 350 to a mixer 352in which chemicals required by the acid peroxide stage are mixed intothe pulp and if it is desirable to pressurize the stage, also oxygen ismixed into the pulp to create the pressure required in the reactor 354.Residual gas is separated from the pulp in the discharger 356 at the topof the reactor 354 and after that the pulp is transferred to a mixer 358in which more chemicals are mixed into the pulp in order to effect thesecond acid peroxide step in a tower 360. The feed into the towers 354and 360 and also to a subsequent tower 366 may be carried out preferablyand if desired via a distributing feed apparatus which prevents the pulpflow from channeling in the tower. From the second peroxide tower 360the pulp is discharged via a top discharger 362 to a following mixer 364in which chelating agents are mixed into the pulp and if necessary alsoother chemicals required by the Q stage.

[0128] It should be noted that the acid peroxide step could be run alsoin one tower for example in the way described in connection with FIG. 5in which case the D tower could be used as a P_(a) tower. The peroxidedose in the acid stage is of the order 1-30 kg/ADMT, preferably 5-20kg/ADMT.

[0129]FIG. 7 illustrates a preferred embodiment of a process arrangementaccording to the invention. FIG. 76 illustrates a preferred way ofeffecting an AP bleaching stage whereby the residual peroxide isutilized. The pulp comes from a tower 410 in which it has been treatedwith a complex former in order to remove harmful metals and to provide afavourable metals profile in the pulp. Acid is added to the pulp in amixer 412, a washer 414 or a pump 416 in order to adjust the pH for theacid treatment according to the invention. In the press 414 which ispreferably a washing press the consistency of the pulp is raised to therange of 20-40 %. The pulp is directed by a pump 416 to a distributor418 which ensures that the pulp is evenly distributed over the wholecross-sectional area 420 of the acid tower whereby the pulp column risesevenly upwards minimizing the risk of channeling. The temperature of thepulp is adjusted to be appropriate either by adding steam to the pulp inthe pipe line or by raising the temperature indirectly in connectionwith the pipe line to in the tower 420 itself. The tower has beendimensioned for the retention time required by the acid treatment. Thetop of the tower 420 has been provided with a top separator 422 withwhich the pulp is preferably discharged to a mixer 424 by means of whichperoxide and if necessary other chemicals such as alkali is/are mixedinto the pulp to raise the pH of the pulp for the peroxide step. Theperoxide and the other chemicals possibly needed may be added also inthe top discharger 422 whereby a separate mixer is not needed for thispurpose.

[0130] Pulp is guided from the mixer 424 to a P tower 426 either in themanner described via one feed connection or via several feed connectionsby applying the distributor 418 described in connection with the tower420. The pulp is discharged from the tower 426 via a top discharger 428to a filter 430 in a way corresponding to the one described inconnection with the tower 420. The peroxide-containing filtrate obtainedfrom this filter is used in the dilution of the pulp to a consistency of10-20% for the P tower 426 by adding the filtrate to the pulp in a mixer424 of the feed line 434. In this way the residual peroxide obtainedfrom the P tower may be utilized although there is no washing pressbetween the A tower and the P tower into which the filtrate could bedirected if the system would include an intermediate wash between thetowers.

[0131] It must be borne in mind that only a few preferred embodiment ofthe invention have been presented above and that real scope ofprotection of the invention is determined by the appended patent claims,only.

1. A method of treating pulp in connection with bleaching of pulpaccording to which method pulp is at first digested and if necessaryoxygen delignified in order to decrease the Kappa number to less than24, preferably less than 14, and after that treated in an acid stage ata pH of 2-5 and at a temperature of 75-130° C. in order to decrease theKappa number by 2-9 units, characterized in that the acid treatment iscarried out in at least one acid tower whereby the retention time t minin the acid treatment is 30-300 minutes and the treatment temperature T°C. is T_(min)<T<T_(max) in which$T_{\min} = {\frac{10517}{24 + {\ln \left( {2t} \right)}} - 273}$

and T _(max) =T _(min)+23° C. in order to prevent the strengthproperties of the pulp from being deteriorated in the acid stage, andthat the acid treatment has been combined into the same treatment stagewith one or several bleaching or treatment chemical/s, which treatmentwith one or several chemical/s is carried out in at least one secondtreatment tower without an intermediate wash after the acid treatment.2. A method as claimed in claim 1 , characterized in that the feed ofpulp into the treatment tower/towers and/or the discharge therefrom iscontrolled so that homogenous flow properties are maintained in thetower so as to have a substantially constant retention time of the pulpin the tower/towers.
 3. A method as claimed in claim 1 or 2 ,characterized in that the acid treatment has been combined into the sametreatment stage with at least one of the following: peroxide, chlorinedioxide, ozone, chlorine, acid peroxide, chelating agent.
 4. A method asclaimed in claim 1 or 2 , characterized in that the acid treatment hasbeen combined into the same treatment stage with chelating agent and atleast one of the following: peroxide, chlorine dioxide, ozone, chlorine,acid peroxide.
 5. A method as claimed in claim 2 , characterized in thatthe retention time is 45-150 minutes.
 6. A method as claimed in any ofthe preceding claims, characterized in that the acid treatment is usedin any of the following bleaching sequences or in the following partialbleaching sequences: Digestion—O-AD-ZD Digestion—O-AD_(N)-DDigestion—O-AD-D Digestion—O-ADQ-O_(p)-D Digestion—O-ADQ-O_(p)-ZDDigestion—O-ADQ-P Digestion—O-AD-O_(p)-D Digestion—O-AD-O_(p)-ZDDigestion—O-AD-P Digestion—O-AP_(a)Q-O_(p)-DDigestion—O-AP_(a)Q-O_(p)-ZD Digestion—O-AP_(a)Q-PDigestion—O-AQ-ZP_(a)Q-P Digestion—O-AP_(a)Q-O_(p)-ZDDigestion—O-P_(a)A-ZD Digestion—O-AP_(a)-D Digestion—O-P_(a)A-DDigestion—O-P_(a)AQ-P Digestion—O-AP_(a)Q-P Digestion—O-AQ-P_(a)Q-PDigestion—AD-ZD Digestion—AD_(N)-D Digestion—AD-D Digestion—ADQ-O_(p)-DDigestion—ADQ-O_(p)-ZD Digestion—ADQ-P Digestion—AD-O_(p)-DDigestion—AD-O_(p)-ZD Digestion—AD-P Digestion—AP_(a)Q-O_(p)-DDigestion—AP_(a)Q-O_(p)-ZD Digestion—AP_(a)Q-PDigestion—AP_(a)Q-O_(p)-ZD Digestion—P_(a)A-ZD Digestion—AP_(a)-DDigestion—P_(a)A-D Digestion—P_(a)AQ-P Digestion—AP_(a)Q-PDigestion—AQ-P_(a)Q-P Digestion—AQ-ZP_(a)Q-P Digestion—O-AQ-ZDQ-PDigestion—AQ-ZDQ-P Digestion—O-AQ-ZQ-P Digestion—O-AQ-PDigestion—O-AQ-O-ZQ-P Digestion—AQ-ZQ-P Digestion—AQ-PDigestion—AQ-O-ZQ-P Digestion—Q-O-AP_(a)Q-P Digestion—O-Q-APDigestion—O-Q-PA Digestion—O-Q-AP_(a) Digestion—O-Q-P_(a)ADigestion—Q-AP Digestion—Q-PA Digestion—Q-AP_(a) Digestion—Q-P_(a)A inwhich sequences the order of the steps in the stages comprising severalsteps is changeable and in which the following abbreviations have beenused: A=a hot acid treatment in a raised temperature, O=oxygentreatment, O_(p)=an oxygen treatment intensified with peroxide, D=achlorine dioxide treatment in which DA means two successive towers D andA without an intermediate wash; this may be referred to also by D/A;also abbreviations AD, AP, PA, AZ, ZA, AP_(a), P_(a)A, AQ and ZQindicate the corresponding situation; D_(N)=chlorine dioxide treatmentwith neutralization, P=peroxide treatment, either atmospheric orpressurized with oxygen, P_(a)=acid peroxide stage usually performedwith peracid, P_(n)=several successive peroxide treatment stages, eitheratmospheric or pressurized with oxygen, E=alkaline stage, Z=ozonetreatment, Q=chelating agent treatment, and “-”=wash between stages 7.An apparatus in connection with treatment of pulp characterized in thatat least one bleaching stage of a sequence comprises at least one acidtower (18, 118, 318, 348, 420) provided with means guiding the pulp feed(20, 24: 120, 124; 316, 320; 346, 350; 418, 422) so that the pulp isdistributed in the feed evenly, and at least one second treatment tower(324, 354, 426) for treatment with some other chemical and that the pulpis discharged from the acid tower so that the retention time, t min, ofthe pulp in the acid tower is substantially constant, 30-300 minutes,and the temperature T° C. is T_(min)<T<T_(max) in which and T _(max) =T_(min)+23° C.$T_{\min} = {\frac{10517}{24 + {\ln \left( {2t} \right)}} - 273}$


8. An apparatus as claimed in claim 7 , characterized in that it hasbeen provided with means (22; 114) for heating the pulp prior to thetower or in the feed end of the tower.
 9. An apparatus as claimed inclaim 7 , characterized in that it has been provided with means (12) forcontrolling the pH of the pulp prior to the tower.
 10. An apparatus asclaimed in claim 7 , characterized in that it comprises an acidtreatment tower (420) and a peroxide treatment tower (426) and thefiltrate connection (432) of the filter (420) following the peroxidetower (426) has been connected to the pulp feed line (434) of theperoxide tower (426).
 11. An apparatus as claimed in claim 7 ,characterized in that the acid tower (18) is an upwards flow tower. 12.An apparatus as claimed in claim 7 , characterized in that the acidtower (118) is a downwards flow tower.
 13. An apparatus as claimed inclaim 7 , characterized in that two acid towers (218, 232) have beenarranged in flow connection with each other for effecting acidtreatment.
 14. An apparatus as claimed in claim 13 , characterized inthat means (222) have been provided between the towers for mixingchemicals into the pulp.
 15. An apparatus as claimed in claim 13 or 14 ,characterized in that one of the towers (218) is an upwards flow towerand the other one (323) is a downwards flow tower.